As conventional sources of natural gas in North America decline while demand for this energy resource continues to grow, coal bed methane (CBM) has been identified as a viable alternative energy source. CBM is aggressively being extracted from multi-zone wellbore formations, and during production of these formations, downhole tools are used to deliver pressurized fluid to stimulate CBM production. In particular, the tool is set within the wellbore to isolate a formation zone, and pressurized nitrogen, or another type of fracturing fluid, is pumped through the tool into the isolated formation zone. The pressurized fluid acts to open or expand “cleats” within the coal seam, thus forming a communication channel through which the CBM can flow into the cased wellbore and then up to the surface.
Fracturing multi-zone CBM wellbore formations is often performed using downhole cup-style straddle-packers. Typically, pressurized nitrogen is pumped through a work string, such as coiled tubing, once these cup-style straddle-packers are set at a particular location within the wellbore. After fracturing a zone, it may be necessary to allow the pressurized formation to bleed down from the applied treatment pressure in order to unseat the cups and allow movement of the straddle-packer to the next zone to be fractured. The time required for this bleed down to occur may be 20 minutes, for example. Because many CBM wellbores have multiple zones to fracture, such as 15 to 20 zones, the total time waiting for formation bleed down to occur can be significant and increases the cost of fracturing the wellbore. As an alternative to waiting for the formation to bleed down, the pressurized fluid contained in the work string may be vented to the surface. This, however, wastes volumes of pressurized fluid that could otherwise be usefully injected into the CBM formations, thereby also increasing the cost of fracturing.
Besides the costs associated with venting pressurized fluid, and the time delays associated with waiting to move conventional straddle-packers, the cup-style sealing elements also have operational limits. As the demand for natural gas continues to rise, it has become necessary to drill deeper wellbores, and therefore, fracture formation zones at greater depths. As wellbore depths increase, cup-style sealing elements reach their operational pressure limits and no longer work reliably. Furthermore, the rubber material of the cups is incompatible with acids and other chemicals that may be contained in some wellbore servicing fluids. Even assuming the rubber cups are suitable for use operationally, venting of a pressurized fluid containing acids or chemicals to the surface may be prohibited due to environmental regulations. Where no such prohibition exists, repeated venting of a pressurized fluid containing acid or chemicals is still undesirable, as such venting can be expensive.
Therefore, due to the time and the increased operational cost associated with moving and re-seating typical cup-style straddle-packers during fracturing of multi-zone CBM well formations, the costs associated with venting pressurized fluid to the surface, the inability of cup-style sealing elements to function reliably at greater wellbore depths, and the incompatibility of rubber cups with acids and other chemicals, a need exists for a downhole tool designed for such operations. Specifically, a need exists for a straddle-packer assembly that reduces the time between fracturing multiple zones, does not require venting of pressurized fluid to the surface, is operational at greater wellbore depths, and is compatible with fluids containing acids and other chemicals.